Primary alkaline electrochemical cells typically have an anode comprising zinc active material, an alkaline electrolyte, a cathode comprising manganese dioxide active material, and an electrolyte permeable separator film, typically of cellulose or cellulosic and polyvinylalcohol fibers, between anode and cathode. Such cell may be designated a Zn/MnO2 cell. The cathode may also contain nickel oxyhydroxide (NiOOH) active material in place of manganese dioxide or in admixture with manganese dioxide. Such cell containing predominantly nickel oxyhydroxide as the cathode active material, may be designated a Zn/NiOOH cell. The alkaline cell casing typically has a cylindrical shape, for example, commonly available in standard cell sizes AAAA (42×8 mm), AAA (44×9 mm), AA (49×12 mm), C (49×25 mm) and D (58×32 mm) size.
In the Zn/MnO2 cell the cathode typically comprises a mixture of manganese dioxide, graphite, alkaline electrolyte normally aqueous potassium hydroxide, and optionally a small amount of binder material, such as polyethylene binder. The manganese dioxide used in the cathode is preferably electrolytic manganese dioxide (EMD) which is made by direct electrolysis of a bath of manganese sulfate and sulfuric acid. The EMD is desirable since it has a high density and high purity. The electrical conductivity of EMD is fairly low. An electrically conductive material is added to the cathode mixture to improve the electric conductivity between individual manganese dioxide particles. Such electrically conductive additive also improves electric conductivity between the manganese dioxide particles and the cell housing, which also serves as cathode current collector. Suitable electrically conductive additives can include, for example, conductive carbon powders, such as carbon blacks, including acetylene blacks, flaky crystalline natural graphite, flaky crystalline synthetic graphite, including expanded or exfoliated graphite. The resistivity of graphites such as flaky natural or expanded graphites can typically be between about 3×10−3 ohm-cm and 4×10−3 ohm-cm.
Alkaline cell cathode for cylindrical shaped cells are normally formed in the shape of disks having a hollow central core. (The term cathode disks as used herein may also be referenced as cathode pellets or tablets.) The top and bottom surfaces of the disk are flat with cylindrical surface therebetween. A plurality of the disks are typically inserted into the cell casing and stacked one on top of the other, for example, as shown in representative U.S. Pat. No. 6,251,539 B1 for Zn/MnO2 cells and in U.S. Pat. No. 7,273,680 B2 for Zn/NiOOH cells. The hollow central core of the disks, are bounded by the cathode disk inside surface running along the disk's central longitudinal axis. The cathode disk's inside surface is typically of cylindrical shape, but may also be other curvilinear shape either regular or irregular, for example, as shown in U.S. Pat. No. 6,514,637 B2. After the cathode disks are inserted into the cell casing a separator sheet is inserted to line the inside surface of cathode disks, that is, to line the disks' hollow core. Zinc anode material is then supplied, typically in the form of a gelled zinc slurry, to fill the hollow core of the cathode. For example, the zinc particles can be admixed with conventional gelling agents, such as sodium carboxymethyl cellulose or the sodium salt of an acrylic acid copolymer, and alkaline electrolyte, normally aqueous potassium hydroxide. The gelling agent serves to suspend the zinc particles and to maintain them in contact with one another. Thus, the filled cell has cathode in electrical contact with the casing housing. An elongated current collector is normally inserted into the anode material. The elongated current collector is in electrical contact with and end cap assembly (insulated from the cell casing). The end cap assembly is crimped over the cell casing to close the cell as shown, for example, in the above cited references U.S. Pat. No. 6,251,539 and U.S. Pat. No. 7,273,680.
The cathode disks are made by inserting the cathode mixture into a die cavity and activating a punch assembly to compress the cathode mixture while in the die cavity. The cathode mixture may be compacted between an upper punch (first punch) and a lower punch (second punch) which form a part of the punch assembly. In the compaction process the upper punch presses down onto the surface of the cathode mixture while the lower punch moves upwards or remains stationary. The compacted cathode disks are ejected from the die by action of a lower plunger which presses upwards onto the disk's bottom surface, thereby lifting the disk out of the die.
A longstanding problem associated with forming such cathode disks for alkaline cells is that as the disk is being ejected from the die, flashing of cathode material tends to form in the small clearance space between the cathode disk and die cavity wall and upper punch. In particular flashing of cathode material can become more pronounced when the upper punch tip's edge wears. As the upper punch tip edge wears the clearance between the punch and the die cavity wall increases. Such increase in clearance creates a void space between disk and die cavity wall which can result in flashing of cathode material as the compacted cathode is being ejected from the die. Such flashing of cathode material causes a thin web or wing of cathode material to attach to and protrude from the disk's top surface and top edge of the disk's outer surface. Such web of material is shown as flashed material 55 and 55a protruding from the top of formed cathode disk 50 in FIG. 10. This results in an uneven or nonuniform top edge of the cathode disk and therefore must be removed before the disk is inserted into the cell.
Moreover, such flashed material breaks off in parts as the cathode disk is being ejected from the die and conveyed and transported to receiving containers. This causes an atmosphere of cathode dust to accumulate in the vicinity of the compaction process. As a safety protection workers may need to wear protected respiratory masks. The dust contains abrasive cathode material which may gradually collect on the surfaces of the punch assembly and peripheral operating equipment causing equipment contamination.
Accordingly, it is desired to improve the method of forming cathode disks for alkaline cells in order to eliminate or else significantly reduce the amount of flashed material which becomes attached to the cathode disk during the disk's formation and compaction.
It is desired to reduce the amount of cathode dust in the atmosphere surrounding the cathode compaction process and compaction of other materials thereby improving air quality in the work environment.